IMO training in "sweetspot" as one or two of the three hard workout you do a week is excellent when mixed with long/slow.
Training in "sweetspot" as your dominant training (as proposed in some cycling training plans) is not optimal. But is significantly better than no training, its just not as good as other approaches.
The science behind why I think this is in this video (just ignore the snarky comments, the science is mainstream). https://www.youtube.com/watch?v=C0n-nnRbFBs
An example study: "Six weeks of a polarized training-intensity distribution leads to greater physiological and performance adaptations than a threshold model in trained cyclists" Threshold is sweetspot. Study weekly workout time was 6-7 hours, with the sweetspot group doing slightly more hours per week to get the total work done equivalent https://pubmed.ncbi.nlm.nih.gov/23264537/
Another study (on ironman competitors, zone 1= long/slow, zone 2 = threshold=sweetspot): "Training-Intensity Distribution During an Ironman Season: Relationship With Competition Performance" "Conclusions: While athletes perform with HR mainly in zone 2, better performances are associated with more training time spent in zone 1. A high amount of cycling training in zone 2 may contribute to poorer overall performance." https://journals.humankinetics.com/view ... e-p332.xml
Another study: "Does Polarized Training Improve Performance in Recreational Runners?" "Conclusions: Polarized training can stimulate greater training effects than between-thresholds training in recreational runners." https://journals.humankinetics.com/view ... e-p265.xml
Studies are never perfect. The best training for you is the training you're willing to do.